The generation of electricity by thermophotovoltaic (TPV) devices has been an area of intense research in the past few years. Generally, TPV systems combust natural gas or other fossil fuels to thermally stimulate the emission of photons (i.e. light) from an emitter structure. The light generated by the emitter structure is absorbed by photocells which in turn generate electricity. Several U.S. Patents describe TPV devices, for example see U.S. Pat. Nos.: 4,584,426; 4,597,734; 4,776,895; 4,906,178; 5,137,583; 5,281,131; 5,356,487; 5,360,490; 5,383,976; 5,403,405; 5,439,532; 5,500,054; and, 5,503,685.
Presently there are several limitations to the development of highly efficient TPV devices including: (1) accommodating the extremely high operating temperatures without the emitter structure melting or degrading; (2) increasing the photon content of specific peak spectral emissions from the emitter; (3) reducing the capital cost of TPV units verses the capital cost of conventional electricity generating means; and, (4) reducing the photovoltaic current collection or power density limitations of the photocells.
The development of high temperature "superemissive" ceramic materials for use in the emitter structure has been proposed as a solution to the first three of the above mentioned limitations.
The term "superemissive material" as used herein, refers to a material that when heated above a threshold temperature, emits photons in relatively narrow and discrete spectral bands. In contrast, blackbody or greybody materials, when heated emit photons in broad spectral bands, the peak wavelength of which depends on the temperature to which-the material has been heated. Examples of superemissive materials include: rare earth oxides, and mixtures of rare earth oxides. Especially useful for the manufacturing of TPV emitter structures are superemitter ceramic fibers of the rare earth oxides and mixtures thereof.
An example of the use of rare earth oxide superemitter fibers in a TPV device is contained in U.S. Pat. No. 5,356,487 in which Goldstein et al. disclose the use of superemissive burners to improve the efficiency of electrical generation in TPV devices. The disclosed burner structures are made using a "relic" process and tend to be fragile and rigid once formed. Essentially, the same method is disclosed by Goldstein in U.S. Pat. Nos. 4,776,895; 4,906,178; and 5,400,765.
Heating metal oxide compositions to generate light is an old technology and is disclosed in several pre-1900 patents, for example see U.S. Pat. Nos.: 359,524; 409,529; 563,524; 575,261; or, 614,556. The principle focus of this early work involves compositions containing thorium oxide with various amounts of other metal oxides being formed into mantles for use in gas lighting. The role of the additional oxides is to change the "color" of the light emitted by the mantle and typically include cerium oxide, yttrium oxide, strontium oxide, lanthanum oxide, uranium oxide, etc. In fact this technology is still in use today in the form of mantles for gas powered camping lanterns. As is well known to those who have used these devices, these mantles are very fragile and cannot be subjected to shock or handling without damage.
The "relic" process, previously mentioned above, is the most commonly used method of making rare earth metal oxide fibers and articles. Generally described, the relic process involves soaking a template made of carbon containing compounds, such as rayon or nylon cloth, in a solution containing a metal salt. The soaked template is dried and heated under carefully controlled conditions to oxidize or "burn-out" the carbon containing compounds that make up the template thus leaving behind a fragile metal oxide structure. The physical properties of the final product are influenced by a variety of factors such as, the metal salts used and their concentration in solution, the duration of the soaking time; the selection of the material for the template, the atmospheric and temperature conditions of the "burn-out" step; the atmospheric and temperature conditions of any subsequent heat treatments. Most importantly, the relic process produces a rare earth metal oxide fiber that has taken the shape and form of the template fibers. Typically these rayon and nylon fibers are short, irregular in shape and have variable surface morphologies. Therefore, although the relic process is easy to carry out, the quality of the final product is of variable quality and lacks uniformity and strength needed for TPV applications.
Therefore, there is a continuing need for new methods of making rare earth oxide superemissive materials that are suitable for use in the emitter structures of TPV devices. These superemissive materials should be able to withstand long term exposure to the high temperature, oxidizing environment encountered in the TPV combustion chamber; emit photons within a narrow wavelength distribution; be able to withstand the rigors of handling and transporting the TPV device; and, yet be simple and economical to make.